Galuskinite, Ca_7(SiO_4)_3(CO_3), a new skarn mineral from the Birkhin gabbro massif, Eastern Siberia, Russia

摘要

In addition to spurrite, Ca_5(SiO_4)_2(CO_3), and tilleyite, Ca_5(Si_2O_7)(CO_3)_2, galuskinite, Ca_7(SiO_4)3(CO_3), is the third mineral in the CaO-SiO_2-CO_2 ternary system. Galuskinite, monoclinic, space group P2_1/c (a = 18.79, b = 6.72, c = 10.47 A, p = 90.79°, V = 1322 A3, Z = 4), occurs in thin veins which cut calcio-olivine, γ-Ca_2SiO_4, skarn with larnite, β-Ca_2SiO_4, relics. Pavlovskyite, Ca_8(SiO_4)_2(Si_3O_(10)), and dellaite, Ca_6(Si_2O_7)(SiO_4)(OH)_2, form a margin between the veins and the calcio-olivine skarn. The sanidinite facies high-temperature skarn formed ~500 Ma ago when gabbroid rocks of the Birkhin complex (Baikal area, Eastern Siberia, Russia) intruded and contact-metamorphosed limestone xenoliths. Galuskinite is a retrograde product of skarn alteration and has neither been described from cement clinker production processes nor from studies of the CaO-SiO_2-CO_2 system. The crystal structure of galuskinite, refined from single crystal X-ray data to R_1 = 3.1%, has a modular character. One may define a polysomatic series with spurrite and larnite as endmembers and galuskinite as a 1:1 polysome built from regular alternating spurrite and larnite modules. Differences between the X-ray powder patterns of galuskinite and spurrite are most obvious in the low 9 region. Galuskinite is named after the Russian mineralogists Irina O. Galuskina and Evgeny V. Galuskin, Faculty of Earth Sciences, University of Silesia, Poland, for their outstanding contributions to skarn mineralogy.